There was a good example of such a preservation era layout featured in Railway Modeller some time ago (Probably about 15 years ago or so), that was based on Great Western practice and the Paignton and Dartmouth Railway in particular IIRC. I can't recall having seen a layout depicting a preserved railway at any exhibitions before, though that doesn't mean that I haven't and have just forgotten.
Additionally, my layout is intended to represent a preserved line: Modelling preserved railways certainly offers a lot of potential and the chance to be less than prototypical with the juxtaposition of different train classes, train formations and other features, but I try to be sensible with it. My layout displays a branch line/industrial ethos, as I find that using smaller locomotives and shorter trains makes a layout seem larger as well as physically maximising what can be done with a limited amount of space. I like to keep the layout "fun" as well, so when I say "smaller locomotives" there are a goodly number of the various budget Hornby 0-4-0 steam and diesel outline locos in amongst the more realistic Pannier Tanks, J94 and Bachmann J72. Mainly as a by-product of being a member of the Hornby Collectors Club for several years: I would never voluntarily have amassed such a large collection of 0-4-0 types otherwise! Trains lengths are typically three carriages (That's the maximum that my bay platform can hold, even though the through platforms are considerably longer), formed of mark 1s or pre-nationalisation stock that is very much au fait with real preserved railway practice. There's also a fair number of the Hornby four wheel carriages, in four or five carriages rakes: Once again, not particularly accurate detail wise but derivatives of the simple four wheel design could be found trundling up rural backwater branch lines and around industrial networks.
Modelling a preserved scene also gives me the opportunity to make use of heritage DMUs now and again: I have a Lima class 101, and intend to get my hands on a 110 and Bachmann 108 in the future. I try to draw the line on the diversity of my rolling stock there: Clearly, a big loco like a "Duchess" or a "Western" would look massively incongruous at the head of just three carriages and navigating some fairly tight curves, but I do fall prey to temptation every now and again: I mean, I do have a "Western" but it only tends to appear to occasionally head my five carriage Pullman rake, which also probably shouldn't be there but I couldn't resist the draw of the current Hornby Pullmans when they were first released, and plenty of preserved railways have a Pullman rake of sorts don't they?
I've also reasoned that since I could never run an A3 or A4 for the reasons I've mentioned above, then surely I could compromise for an example of their smaller but just as attractive cousin, the V2, but I have yet to act on that impulse and that is as far as I would go in the "big loco" stakes, honest.
Hey, it's my layout, and I'll do what I want with it!
